1. Field of Invention
The present invention relates to a display device. More particularly, the present invention relates to a method of driving a display device.
2. Description of Related Art
Dynamic recording of documentary through film has a long history. With the invention of cathode ray tube (CTR) and broadcasting equipment, television has become an indispensable electronic device in almost every family. In the electronic industry, CRTs are also used as monitors for desktop computers. However, CRT is now gradually being phased out due to radiation hazards and bulkiness of the CRT body that needs to house an electron gun.
Because of radiation hazards and bulkiness, flat panel displays have been developed. The types of flat panel displays now include liquid crystal display (LCD), field emission display (FED), organic light emitting diode (OLED) and plasma display panel (PDP).
Organic light emitting diode (OLED) is sometimes referred to as organic electroluminescence display (OELD). OLED is a type of self-illuminating device arranged to form a matrix of points. Each OLED is driven by a low DC current to produce light having a high luminance and contrast. The OLED also has a high operating efficiency and carries very little weight. Moreover, the OLED may emit light within a range of colors including the three primary colors red (R), green (G), blue (B) and white light. Consequently, OLED is currently the most actively developed type of flat panel display. Aside from high-resolution, lightweight, active illumination, quick response and energy saving capacity, the advantages of OLED further include a large viewing angle, good color contrast and low production cost. Currently, the OLED has many applications such as a light source at the back of a LCD or indicator panel in a mobile phone, a digital camera, a personal digital assistant (PDA) and so on.
According to the driving method, OLED may be classified into two major types, namely, a passive matrix driven type and an active matrix driven type. The passive matrix driven OLED has a simpler structure and does not use any thin film transistor (TFT). Hence, the passive matrix driven OLED is easier and less expensive to produce. However, the passive matrix driven OLED has a lower resolution and consumes a lot of electrical energy if the display area is large. On the other hand, the active matrix driven OLED is suitable for fabricating large display panels. The active matrix driven OLED panel has a wide viewing angle, illuminates brightly and responds quickly to control signals. Nevertheless, the active matrix driven OLED panel is slightly more expensive to produce.
According to the driving mode, flat panel displays can be categorized as voltage driven or current driven. The voltage driven mode is commonly employed in a thin film transistor liquid crystal display (TFT-LCD). To operate a voltage driven TFT-LCD, different voltages are fed to data lines so that different color gray scales are produced and a full coloration is obtained. The voltage driven TFT-LCD is relatively stable and cheap to manufacture. The OLED is a type of current driven display. To operate an OLED display, different currents are fed to data lines so that different color gray scales are produced and a full coloration is obtained. Before operating this type of current driven pixel, however, new circuits and ICs must first be developed. The cost of developing new circuits and ICs is high. Thus, if the voltage-driven circuit of a TFT-LCD can somehow be used to drive the OLED, production cost will be very much lowered. However, when the voltage-driven circuit of TFT-LCD is used to drive the OLED, threshold voltage of the driving TFT may drift leading to an increase in threshold voltage after extended operation. The drain current for a thin film transistor operating in the saturated region is given by the formula:Id=(½) Ã□ÂμnÃ□CoxÃ□(W/L)Ã□(Vgs−Vth)2,where the electron mobility Âμn and the gate capacitance per unit area Cox are constant values, Vth is the threshold voltage of the thin film transistor (TFT), W is the width of the TFT channel and L is the length of the TFT channel. According to the formula, the driving current flowing from the drain terminal to the source terminal of the TFT will decrease when the threshold voltage is increased. Since the driving current is used for driving the OLED to emit light, a lowering of the driving current leads to a reduction of OLED luminance.
FIG. 1 is a diagram showing a driving circuit for driving a voltage-driven OLED inside one pixel of a display device. As shown in FIG. 1, the pixel 10 includes a voltage-driven circuit 102 and an OLED 104. The voltage-driven circuit 102 further includes a first transistor (TFT1) 106, a capacitor (C) 108 and a second transistor (TFT2) 110. The second transistor (TFT2) 110 is a driving thin film transistor for producing a driving current to the OLED 104 so that the OLED 104 lights up. The drain terminal of the first transistor (TFT1) 106 is connected to a data voltage (Vdata) terminal. The source terminal of the first transistor (TFT1) 106 is connected to the first terminal of the capacitor (C) 108 and the gate terminal of the second transistor (TFT2) 110. The drain terminal of the second transistor (TFT2) 110 is connected to a power supply that provides a voltage (VDD). The source terminal of the second transistor (TFT2) 110 is connected to the positive terminal of the OLED 104. In general, the voltage (VDD) is a positive voltage. The second terminal of the capacitor (C) 108 is connected to a power supply that provides a reference voltage (Vref) The negative terminal of the OLED 104 is connected to a ground terminal.
FIG. 2 is a diagram showing the waveform of various voltages for operating the voltage-driven circuit 102 in FIG. 1. Timing diagrams registered in FIG. 2 include VDD, Vscan, Vdata and the gate voltage V2g of the second transistor (TFT2) 110. As shown in FIG. 2, when the voltage Vscan is set to a high voltage level, the first transistor (TFT1) 104 will conduct. Note that the time interval between the appearance of a high voltage level and a low voltage level in Vscan is referred to as a time frame (labeled T in FIG. 2). In general, a time frame is 1/60 second or a frequency of 60 Hz and a time frame constitutes a pixel image. When the voltage Vscan is at a high voltage level, Vdata is at a high voltage level so that a positive voltage is applied to the node point V2g. Hence, the gate voltage V2g rises gradually. A rise in the gate voltage V2g leads to the accumulation of trapped charges in the gate oxide layer of the second transistor (TFT2) 110. Hence, the threshold voltage of the second transistor (TFT2) 110 drifts to a higher value. Ultimately, the driving current flowing from the drain terminal to the source terminal of the second transistor (TFT2) 110 is lowered and luminance of the OLED 104 is reduced.